The invention relates to a method of operating an internal-combustion engine which has at least one cylinder with a combustion chamber assigned to it.
A method of the above-mentioned type is known, for example, from German Patent Document DE 102 50 121 A1 wherein the exhaust valve is opened during the compression stroke carried out by the piston, so that unburnt fuel and air is released from the cylinder. The fuel oxidizes in the exhaust treatment device with the air which leads to the release of heat thereby generating a desired temperature increase in the exhaust treatment device. This metered addition of fuel into the exhaust pipe is, however, not completely without problems and requires a very effective control in order to avoid any disadvantageous effects.
From International Patent Document WO 03/104622 A1, a control method is known for regenerating a particle filter in an exhaust system in the case of which the timing of the charge cycle valves is to be changed during the intake stroke in order to increase the temperature in the exhaust pipe to a temperature which is suitable for regenerating the particle filter. However, by means of the solution described therein, the achieved effects are insufficient.
International Patent Document WO 01/29383 A1 describes a method of influencing the exhaust gas temperature in an internal-combustion engine in which, in order to raise the exhaust gas temperature, the combustion chamber exhaust valve is opened earlier. For this purpose, conventional camshaft adjusting devices are used by means of which an adjustment of the camshaft by typically approximately 10 to 20° can be achieved. The described method is only suitable for internal-combustion engines with a variable compression ratio, which, however, generates very high expenditures and thus extremely high costs.
Japanese Patent Document JP 2002295215 A describes a system for increasing the exhaust gas temperature of an internal-combustion engine where the timing of the exhaust valve can be adjusted by a crankshaft angle of approximately 20 to 40° in the early direction. In this case, the exhaust valve is opened in the compression stroke.
In European Patent Document EP 1 389 674A1, a strategy is described according to which the exhaust valve is opened during the intake stroke in order to take in not only cold fresh air but also again take in already burnt hot gas.
However, all of the mentioned methods and systems are not capable of producing a temperature of the exhaust gas leaving the combustion chamber sufficient, for example, for the regeneration of a particle filter.
It is therefore an object of the present invention to provide a method for operating an internal-combustion engine by which, by means of simple devices, an increase of the temperature existing in the exhaust pipe can be achieved which is sufficient for the regeneration of an exhaust treatment device.
In the case of the method according to the invention, the exhaust valve is opened in the area of the upper compression dead center of the piston and thus at a considerably earlier point in time than normally, whereby the hot compressed air or the exhaust gases can escape from the combustion chamber into the exhaust pipe, raise the temperature and in this manner warm up the exhaust treatment device. This permits a regeneration of the exhaust treatment device.
The method according to the invention can advantageously be implemented by means of very simple, for example, hydraulic or mechanical devices, because a corresponding opening of the outlet valve can be carried out by known devices which may already exist on the internal-combustion engine.
It was found to be particularly advantageous with respect to the heat fed into the exhaust pipe for the exhaust valve to be opened at the earliest 40° before and at the latest 40° after the upper compression dead center of the piston.
A synchronization of the ignition of the fuel-air mixture with the opening of the exhaust valve can occur when the fuel-air mixture in the cylinder is ignited at the earliest 40° before the upper compression dead center of the piston and at the latest 40° after the upper compression dead center of the piston.
In the case of multi-cylinder internal-combustion engines, a particularly advantageous embodiment of the method can consist of the fact that at least one of the cylinders is operated in an unfired manner. In this case the temperature in the exhaust pipe is increased because the exhaust valve is opened in the area of the upper compression dead center of the piston, and at least one of the cylinders being operated in a fired manner. In the case of this cylinder, the exhaust valve is held during the compression stroke essentially in its closed position. The air in the unfired cylinders is compressed to a higher temperature and in the area of the upper dead center of the piston, is guided into the exhaust pipe.
By means of such a method, it becomes possible to simultaneously regenerate the exhaust treatment device and nevertheless operate the internal-combustion engine such that it provides sufficient useful power, for example, for operating a motor vehicle. This result in a temperature in the exhaust pipe which is the higher when more cylinders are unfired and are operated with the opening of the exhaust valve according to the invention in the range of the upper compression dead center of the piston. In this case, the fired cylinders have to yield a greater power than in an operating mode in which all cylinders are fired, which advantageously results in a further increase of the exhaust gas temperature.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.